Quincy, while I may be msinterting the situation, to me, it would appear there was no intent to create a joint work. Lyricist simply requested permission to use op's work and create a derivative work yet you stated it is treated as a joint work unless a written agreement states otherwise; why? Why wouldn't the music remain an independent work and the derivative remain music owned by op and additions owned by lyricist?
If it is deemed a joint work wouldn't that give lyricist the right to use only the music (sans lyrics) in other works without permission of op as owner of copyrights of the music?
It is a joint work if the intention of the authors is to create a single copyrighted work. It is a derivative work if a new work is created based on a preexisting work.
What misteraudio wants to do with the copyright ownership, and what the singer wants to do with the copyright ownership, will determine how the work should be registered.
If registered as a joint work with a contract spelling out specific ownership rights (e.g., misteraudio retains 100% of instrumental rights, singer retains 100% of lyrics/vocals, both share revenues from joint work 50-50), this can be done. It will simply be registered as a joint work and an agreement will spell out the specifics. If there is no agreement and the work is registered as a joint work, copyrights in the registered work will be shared equally and all revenues generated by this joint work must also be shared equally.
misteraudio could also license rights to the singer for X amount of dollars (or no dollars), with the license providing whatever it is misteraudion and the singer want and need it to provide. A license could allow the singer rights only to use the instrumental as it is recorded with her lyrics and vocals. The rights to use can be restricted, in other words. She could have 100% of all revenue generated from the work, if this is what they want, or perhaps misteraudio could have 20% or whatever. With limited licensing, misteraudio retains his rights and he can resell or use his instrumental for other purposes. The singer would be limited by the terms of the license.
Contracts are basically whatever two parties agree to, and these agreements can be fashioned to meet the wants and needs of the parties. But the
exclusive rights of a copyright holder cannot be transferred to another without a written and signed agreement (i.e., a copyright holder cannot give to another any or all of his exclusive rights in a work, making another the copyright holder, without a written and signed transfer agreement).
And without any written agreement at all, the law will presume certain things.
So far, no copyrights have been transferred. Both misteraudio and the singer retain their exclusive rights to their own copyrighted material. There has been only an oral agreement between misteraudio and the singer allowing for the singer to use the instrumental in the single recording.
What happens now is essentially up to misteraudio and the singer. There are options. The available options, and their pros and cons, are best personally reviewed by a copyright lawyer.
But I think it is important for
something to be in writing between misteraudio and the singer, to solidify the agreement and its terms, so, if a dispute arises, both parties can look at the written agreement and know exactly where they stand legally.